The present invention relates generally to solenoid actuated valve assemblies and more specifically to a plunger assembly therefor having a preset spring force pre-load.
A problem frequently encountered concerning solenoid actuated valve assemblies is uniformly providing a preset spring force pre-load of the plunger therewithin due to physical inconsistencies among xe2x80x9cidenticalxe2x80x9d springs. It is difficult and expensive to provide springs which all have perfectly consistent spring stiffness, in that the spring constant and/or the spring length may be different over a range related to spring manufacturing tolerances. Yet, solenoid actuated valve assemblies control hydraulic fluid flow based upon regulation of a balance of forces acting on the plunger: the hydraulic fluid force and the magnetic field force of the solenoid versus the spring force. Unless the spring force pre-load can be consistently set, an unacceptably wide range of valve seat operational characteristics will result among xe2x80x9cidenticalxe2x80x9d valve assemblies. Therefore, the problem of uniformly providing preset spring force pre-load among all identical valve assemblies must be solved.
There are several previously known ways of solving the problem of non-uniform spring force pre-load.
A first known method uses a setscrew to adjust spring force pre-load. The spring compression is varied by turning the setscrew to change the spring pocket length. The disadvantage to this method is that the setscrew increases the size of the valve assembly, introduces a potential leak path, negatively affects the magnetic circuit, can unthread over time, and typically requires an end-of-line adjustment.
A second known method uses shims to adjust spring force pre-load. Shims are placed in an oversized spring pocket to shorten the length of the spring pocket and thereby vary the spring force pre-load. The shims do not affect the magnetic circuit, do not change with time, nor introduce new leak paths. However, shims are difficult to work with and the adjustment process is time consuming because of the discrete steps of inserting shims until the target spring force pre-load is met.
A third known method uses in-line precision machining to create a spring pocket which will create the correct spring force pre-load for a specific spring. Precision machining is expensive and time consuming.
Accordingly, there is a clearly felt need in the art for providing valve assemblies all having uniform spring force pre-load which has none of the aforementioned disadvantages.
The present invention provides a uniformly preset spring force pre-load among a number of identical solenoid actuated valve assemblies by uniquely adjusting the spring pocket length of each respective plunger to accommodate a respective spring which is mated thereto. This adjustment is accomplished using a two-component plunger consisting of a plunger body having a central bore and a rod slidably located within the central bore. The rod is press-fit such that slidable movement of the rod relative to the plunger body can only occur if a predetermined minimum of force is applied, which force is considerably more than that capable of being exerted by the compression force of the spring.
The spring pocket includes a portion of the central bore unoccupied by the rod. Accordingly, by pressing the rod a selected distance into the central bore, a spring pocket is obtained which is uniquely correct for a selected spring, in that the selected spring is compressed to a precisely preset spring force pre-load. The adjusted plunger and spring are uniquely assigned to each other to collectively form a customized plunger assembly having a preset spring force pre-load, whereupon a valve assembly is manufacturable therewith having the precisely preset spring force pre-load.
In caring-out the adjustment of the plunger with respect to a specific spring, the rod is pressed into the central bore by a controllable source of force, such as for example provided by a stepper motor, and the spring force pre-load is monitored to ascertain when the desired spring force pre-load is achieved. Monitoring and force control is preferably automatic, but may be manual.
Accordingly, it is an object of the present invention to provide a plunger assembly which has a preset spring force pre-load.
This and additional objects, advantages, features and benefits of the present invention will become apparent from the following specification.